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Climate Change & El Niņo

Introduction

El Niño is the name given to the occasional development of warm ocean surface waters along the coast of equatorial South America. When this warming occurs the usual upwelling of cold, nutrient rich deep ocean water is cut off, drastically reducing fish populations in this part of the Pacific Ocean. Jet streams in the upper atmosphere, storm tracks and monsoons are shifted. El Niño normally occurs around Christmas and lasts usually for a few weeks to a few months. Sometimes an extremely warm event can develop that lasts for much longer time periods. In the early 1990s, a strong El Niño developed in 1991 and lasted until 1995. It is possible that human-induced climate change could bring more frequent and intense El Niño weather patterns. This fact sheet reviews El Niño and the likely effects climate change will have on it.

What is El Niño?

El Niño is Spanish name for "Christ Child", and initially referred to a weak, warm current appearing annually around Christmas time along the coast of Ecuador and Peru and lasting only a few weeks to a month or more. Every three to seven years, an El Niño event may last for many months, having significant economic and atmospheric consequences world-wide. During the past forty years, ten of these major El Niño events have been recorded, the worst of which began in 1997. Previous to this, the El Niño event in 1982-1983 was the strongest.

El Niño: An Ocean Current


The formation of El Niño is linked with the cycling of a Pacific Ocean circulation pattern known as the El Niño Southern Oscillation or ENSO. In a normal year, low atmospheric pressure develops over northern Australia and Indonesia, with an anticyclone or high pressure over the equatorial Pacific. Consequently, trade winds over the Pacific move from east to west. The easterly flow of the trade winds carries warm surface waters westward bring convective storms to Indonesia and coastal Australia. Along the coast of Peru and Ecuador, cold bottom water wells up to the surface to replace the warm water that is pulled to the west.

In an El Niño year, low pressure over northern Australia is replaced by high pressure, whilst air pressure falls over large areas of the central Pacific and along the coast of South America. This change in pressure pattern causes the normal easterly trade winds to be reduced and sometimes reversed. This allows warm equatorial water to flow or "slop" back eastward across the Pacific, and accumulate along the coastlines of Peru and Ecuador. The warm water off the equatorial South American coast cuts off the upwelling of cold deep ocean water. The unusually warm water on the eastern side of the Pacific creates large moisture-laden convection currents in the atmosphere, leading to drastically increased rates of precipitation and flooding. In contrast, the high pressure and cooler waters around Australasia preclude the formation of major rain storms, leading to drought, and sometimes extensive bush fires as the vegetation dries up.

After an El Niño event, weather conditions usually return back to normal. However, in some years the easterly trade winds can become extremely strong and an abnormal accumulation of cold water can occur in the central and eastern Pacific. This event is called a La Niña. The cold La Niña events sometimes (but not always) follow El Niño events.

Climatic Impacts of El Niño

During an El Niño year, tropical rains usually centred over Indonesia and northern Australia shift eastward, influencing atmospheric wind patterns world wide. As well as flooding in Peru and Ecuador, and drought in Indonesia and Australia, possible impacts include a shifting of the jet stream, storm tracks and monsoons, producing unseasonable weather over many regions of the globe. During the El Niño event of 1982-1983, some of the abnormal weather patterns which were observed included drought in Southern Africa, Southern India, Sri Lanka, Philippines, Indonesia and Australia; heavy rain and flooding in Bolivia, Ecuador, Northern Peru and Cuba; and hurricanes in Tahiti and Hawaii. Because El Niño may influence the mid-latitude northern hemisphere jet stream even the weather in Europe and North America can be influenced. The most recent El Niño episode in 1997 and 1998 has brought record high winter temperatures to many areas in Europe including the UK.

Economic Impacts of El Niño

The coast of Peru is one of five major fishing grounds in the world. The abundance of fish is usually supported by the upwelling of cold nutrient rich waters from deeper levels. During non-El Niño years the easterly trade winds drag surface water westward away from the coast allowing the (nutrient-rich) deeper waters to rise. During an El Niño event, the weakening of the easterly trade winds and the thicker layer of warm water off the Peruvian coast prevent the upwelling of deeper water. Consequently, warm nutrient-poor water predominates the region and a decrease in the fish population is observed. A reduction of the fish population reduces the amount of fishmeal produced and exported (by local industry) to other countries for feeding poultry and livestock. If the world's fishmeal supply decreases, more expensive alternative feed sources must be used, resulting in an increase in poultry prices world-wide.

El Niño and Climate Change

Some scientists have begun to investigate whether human-induced climate change could bring more frequent and intense El Niño events, and the accompanying weather patterns which bring devastating floods and droughts to nations surrounding the Pacific Ocean. Some evidence does point to a change in the strength and intensity of the El Niño weather events since the 1970's. While some uncertainties remain as to the cause of intense El Niño episodes, there is increasing scientific concern that human-induced climate change could be a contributing factor. Computer modelling studies have found that a doubling of atmospheric carbon dioxide increases the frequency of El Niño from an average of once in every five years to once in every three years.

Current understanding of El Niño and the climate system does not allow scientists to conclude whether any recent increase in intensity of El Niño is the result of man-made global warming, or whether the observed rise in global surface temperatures is due to a naturally enhanced El Niño - a kind of "chicken and egg" scenario. Certainly, much of the warming seen during 1997 in the northern hemisphere was due to El Niño-caused changes in jet stream patterns, rather than an increase in temperatures due to the increase in greenhouse gases. Equally, whilst increases in atmospheric greenhouse gas concentrations provide a likely candidate for changes in El Niño and other climatic phenomena, other natural causes cannot at this stage be ruled out. Nevertheless, because El Niño has such dramatic climatic and economic consequence on a global scale, the possibility that global warming may increase the frequency and intensity of El Niño events must be considered seriously.

Conclusion

Our understanding of the processes responsible for the development of El Niño is still incomplete. Although researchers have a reasonable understanding of the global weather effects caused by the formation El Niño events, it is not possible to say how man-made changes in climate as a result of increasing greenhouse gas emissions will affect the frequency and intensity of El Niño. What is more certain, however, is that El Niño will continue to have major climatic as well as economic impacts on those societies inhabiting the Pacific rim, as well as large populations of both the developing and developed world further afield.